HVAC Duct Sizing & Airflow CFM Calculator
Size round and rectangular ducts instantly on-site. Calculate required CFM, air velocity (FPM), equivalent dimensions, and friction loss according to ACCA Manual D standards.
Airflow CFM & Duct Dimension Estimator
Adjust airflow volume and target friction rate to calculate duct sizes in real time:
Round Duct Sizing & CFM Capacity Chart
Standard sheet metal and flexible duct sizing guide at 0.10 in. wg / 100 ft friction loss:
| Duct Diameter | Airflow Range (CFM) | Avg Velocity (FPM) | Typical Residential / Light Commercial Use |
|---|---|---|---|
| 5" | 40 - 65 CFM | 450 FPM | Small bathroom, closet supply |
| 6" | 75 - 110 CFM | 500 FPM | Standard bedroom, small office |
| 7" | 120 - 160 CFM | 550 FPM | Master bedroom, living room branch |
| 8" | 180 - 240 CFM | 600 FPM | Large living area, kitchen supply |
| 9" | 260 - 320 CFM | 650 FPM | Open floor plan branch |
| 10" | 350 - 450 CFM | 700 FPM | Main trunk branch (approx. 1 Ton) |
| 12" | 550 - 700 CFM | 750 FPM | Sub-trunk line (approx. 1.5 - 2 Tons) |
| 14" | 800 - 1,000 CFM | 850 FPM | Main trunk duct (2.5 Tons) |
| 16" | 1,100 - 1,400 CFM | 900 FPM | Central return trunk (3 - 3.5 Tons) |
| 18" | 1,500 - 1,900 CFM | 950 FPM | Large commercial/residential main trunk (4 - 5 Tons) |
Ductwork Is Behind 40%+ of HVAC Breakdowns
Undersized ducts create high static pressure, frozen evaporator coils, and premature compressor burnouts.
Evaporator Coil Icing
Undersized return trunks choke airflow below 350 CFM/ton. Coil temperatures plummet below 32°F, freezing condensate and starving the compressor of heat load.
Furnace High Limit Tripping
Restricted supply plenums trap heat inside furnace heat exchangers. High-limit switches trip within 4 minutes of burner ignition (Carrier Code 33, Goodman 4 Flashes).
ECM Blower Overamp & Burnout
Modern constant-CFM ECM blowers ramp up RPM against high static pressure (> 0.8 in. w.g.), pulling excessive amperage until control modules overheat and fail.
Duct Sizing & CFM FAQ
How do I know if a home's ductwork is undersized for a new AC install?
Measure total external static pressure (TESP) with a dual-port digital manometer. If supply + return static exceeds the manufacturer's maximum rating (typically 0.50 in. w.g. for standard furnaces or 0.70 in. w.g. for air handlers), the ductwork is undersized and will restrict required CFM.
Why do flex ducts need to be sized one inch larger than rigid sheet metal?
Flexible ductwork has internal wire helix ridges and sag that create significantly higher friction loss (equivalent to 0.05–0.08 in. wg/100ft vs. smooth galvanized steel at 0.10). An 8-inch flex duct flows approximately the same CFM as a 7-inch smooth round metal duct.
Can I use this duct calculator inside the ACLogics mobile app offline?
Yes. The ACLogics mobile app has this complete duct calculator built directly into the technician toolkit tab, functioning 100% offline with zero internet required.
Field Toolkit
Get the Offline Duct Calculator on Your Phone
Download ACLogics free for iPhone and Android. Try for free — no credit card needed. Size ducts, look up error codes, and diagnose equipment seamlessly in the field.